On Sun, Mar 15, 2026 at 07:18:38PM -0700, Suren Baghdasaryan wrote:
> On Fri, Mar 13, 2026 at 4:58 AM Lorenzo Stoakes (Oracle) <[email protected]>
> wrote:
> >
> > On Fri, Mar 13, 2026 at 04:02:36AM -0700, Usama Arif wrote:
> > > On Thu, 12 Mar 2026 20:27:19 +0000 "Lorenzo Stoakes (Oracle)"
> > > <[email protected]> wrote:
> > >
> > > > Previously, when a driver needed to do something like establish a
> > > > reference
> > > > count, it could do so in the mmap hook in the knowledge that the mapping
> > > > would succeed.
> > > >
> > > > With the introduction of f_op->mmap_prepare this is no longer the case,
> > > > as
> > > > it is invoked prior to actually establishing the mapping.
> > > >
> > > > To take this into account, introduce a new vm_ops->mapped callback
> > > > which is
> > > > invoked when the VMA is first mapped (though notably - not when it is
> > > > merged - which is correct and mirrors existing mmap/open/close
> > > > behaviour).
> > > >
> > > > We do better that vm_ops->open() here, as this callback can return an
> > > > error, at which point the VMA will be unmapped.
> > > >
> > > > Note that vm_ops->mapped() is invoked after any mmap action is
> > > > complete (such as I/O remapping).
> > > >
> > > > We intentionally do not expose the VMA at this point, exposing only the
> > > > fields that could be used, and an output parameter in case the operation
> > > > needs to update the vma->vm_private_data field.
> > > >
> > > > In order to deal with stacked filesystems which invoke inner
> > > > filesystem's
> > > > mmap() invocations, add __compat_vma_mapped() and invoke it on
> > > > vfs_mmap() (via compat_vma_mmap()) to ensure that the mapped callback is
> > > > handled when an mmap() caller invokes a nested filesystem's
> > > > mmap_prepare()
> > > > callback.
> > > >
> > > > We can now also remove call_action_complete() and invoke
> > > > mmap_action_complete() directly, as we separate out the rmap lock logic
> > > > to
> > > > be called in __mmap_region() instead via maybe_drop_file_rmap_lock().
> > > >
> > > > We also abstract unmapping of a VMA on mmap action completion into its
> > > > own
> > > > helper function, unmap_vma_locked().
> > > >
> > > > Additionally, update VMA userland test headers to reflect the change.
> > > >
> > > > Signed-off-by: Lorenzo Stoakes (Oracle) <[email protected]>
> > > > ---
> > > > include/linux/fs.h | 9 +++-
> > > > include/linux/mm.h | 17 +++++++
> > > > mm/internal.h | 10 ++++
> > > > mm/util.c | 86 ++++++++++++++++++++++++---------
> > > > mm/vma.c | 41 +++++++++++-----
> > > > tools/testing/vma/include/dup.h | 34 ++++++++++++-
> > > > 6 files changed, 158 insertions(+), 39 deletions(-)
> > > >
> > > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > > index a2628a12bd2b..c390f5c667e3 100644
> > > > --- a/include/linux/fs.h
> > > > +++ b/include/linux/fs.h
> > > > @@ -2059,13 +2059,20 @@ static inline bool can_mmap_file(struct file
> > > > *file)
> > > > }
> > > >
> > > > int compat_vma_mmap(struct file *file, struct vm_area_struct *vma);
> > > > +int __vma_check_mmap_hook(struct vm_area_struct *vma);
> > > >
> > > > static inline int vfs_mmap(struct file *file, struct vm_area_struct
> > > > *vma)
> > > > {
> > > > + int err;
> > > > +
> > > > if (file->f_op->mmap_prepare)
> > > > return compat_vma_mmap(file, vma);
> > > >
> > > > - return file->f_op->mmap(file, vma);
> > > > + err = file->f_op->mmap(file, vma);
> > > > + if (err)
> > > > + return err;
> > > > +
> > > > + return __vma_check_mmap_hook(vma);
> > > > }
> > > >
> > > > static inline int vfs_mmap_prepare(struct file *file, struct
> > > > vm_area_desc *desc)
> > > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > > index 12a0b4c63736..7333d5db1221 100644
> > > > --- a/include/linux/mm.h
> > > > +++ b/include/linux/mm.h
> > > > @@ -759,6 +759,23 @@ struct vm_operations_struct {
> > > > * Context: User context. May sleep. Caller holds mmap_lock.
> > > > */
> > > > void (*close)(struct vm_area_struct *vma);
> > > > + /**
> > > > + * @mapped: Called when the VMA is first mapped in the MM. Not
> > > > called if
> > > > + * the new VMA is merged with an adjacent VMA.
> > > > + *
> > > > + * The @vm_private_data field is an output field allowing the user
> > > > to
> > > > + * modify vma->vm_private_data as necessary.
> > > > + *
> > > > + * ONLY valid if set from f_op->mmap_prepare. Will result in an
> > > > error if
> > > > + * set from f_op->mmap.
> > > > + *
> > > > + * Returns %0 on success, or an error otherwise. On error, the VMA
> > > > will
> > > > + * be unmapped.
> > > > + *
> > > > + * Context: User context. May sleep. Caller holds mmap_lock.
> > > > + */
> > > > + int (*mapped)(unsigned long start, unsigned long end, pgoff_t pgoff,
> > > > + const struct file *file, void **vm_private_data);
> > > > /* Called any time before splitting to check if it's allowed */
> > > > int (*may_split)(struct vm_area_struct *vma, unsigned long addr);
> > > > int (*mremap)(struct vm_area_struct *vma);
> > > > diff --git a/mm/internal.h b/mm/internal.h
> > > > index 7bfa85b5e78b..f0f2cf1caa36 100644
> > > > --- a/mm/internal.h
> > > > +++ b/mm/internal.h
> > > > @@ -158,6 +158,8 @@ static inline void *folio_raw_mapping(const struct
> > > > folio *folio)
> > > > * mmap hook and safely handle error conditions. On error, VMA hooks
> > > > will be
> > > > * mutated.
> > > > *
> > > > + * IMPORTANT: f_op->mmap() is deprecated, prefer f_op->mmap_prepare().
> > > > + *
>
> What exactly would one do to "prefer f_op->mmap_prepare()"?
I'm saying a person should implement f_op->mmap_prepare() rather than
f_op->mmap(), since the latter is deprecated :)
I think that's pretty clear no?
> Since you are adding this comment for mmap_file(), I think you need to
> describe more specifically what one should call instead.
I think it'd be a complete distraction, since if you're at the point of calling
mmap_file() you're already not implement mmap_prepare except as a compatbility
layer.
I mean maybe I'll just drop this as it seems to be causing confusion.
>
> > > > * @file: File which backs the mapping.
> > > > * @vma: VMA which we are mapping.
> > > > *
> > > > @@ -201,6 +203,14 @@ static inline void vma_close(struct vm_area_struct
> > > > *vma)
> > > > /* unmap_vmas is in mm/memory.c */
> > > > void unmap_vmas(struct mmu_gather *tlb, struct unmap_desc *unmap);
> > > >
> > > > +static inline void unmap_vma_locked(struct vm_area_struct *vma)
> > > > +{
> > > > + const size_t len = vma_pages(vma) << PAGE_SHIFT;
> > > > +
> > > > + mmap_assert_locked(vma->vm_mm);
>
> You must hold the mmap write lock when unmapping. Would be better to
> assert mmap_assert_write_locked() or even vma_assert_write_locked(),
> which implies mmap_assert_write_locked().
I'm not sure why we don't assert this in those paths.
I think I assumed we could only assert readonly because one of those paths
downgrades the mmap write lock to a read lock.
I don't think we can do a VMA write lock assert here, since at the point of
do_munmap() all callers can't possibly have the VMA write lock, since they are
_looking up_ the VMA at the specified address.
But I can convert this to an mmap_assert_write_locked()!
>
> > > > + do_munmap(vma->vm_mm, vma->vm_start, len, NULL);
> > > > +}
> > > > +
> > > > #ifdef CONFIG_MMU
> > > >
> > > > static inline void get_anon_vma(struct anon_vma *anon_vma)
> > > > diff --git a/mm/util.c b/mm/util.c
> > > > index dba1191725b6..2b0ed54008d6 100644
> > > > --- a/mm/util.c
> > > > +++ b/mm/util.c
> > > > @@ -1163,6 +1163,55 @@ void flush_dcache_folio(struct folio *folio)
> > > > EXPORT_SYMBOL(flush_dcache_folio);
> > > > #endif
> > > >
> > > > +static int __compat_vma_mmap(struct file *file, struct vm_area_struct
> > > > *vma)
> > > > +{
> > > > + struct vm_area_desc desc = {
> > > > + .mm = vma->vm_mm,
> > > > + .file = file,
> > > > + .start = vma->vm_start,
> > > > + .end = vma->vm_end,
> > > > +
> > > > + .pgoff = vma->vm_pgoff,
> > > > + .vm_file = vma->vm_file,
> > > > + .vma_flags = vma->flags,
> > > > + .page_prot = vma->vm_page_prot,
> > > > +
> > > > + .action.type = MMAP_NOTHING, /* Default */
> > > > + };
> > > > + int err;
> > > > +
> > > > + err = vfs_mmap_prepare(file, &desc);
> > > > + if (err)
> > > > + return err;
> > > > +
> > > > + err = mmap_action_prepare(&desc, &desc.action);
> > > > + if (err)
> > > > + return err;
> > > > +
> > > > + set_vma_from_desc(vma, &desc);
> > > > + return mmap_action_complete(vma, &desc.action);
> > > > +}
> > > > +
> > > > +static int __compat_vma_mapped(struct file *file, struct
> > > > vm_area_struct *vma)
> > > > +{
> > > > + const struct vm_operations_struct *vm_ops = vma->vm_ops;
> > > > + void *vm_private_data = vma->vm_private_data;
> > > > + int err;
> > > > +
> > > > + if (!vm_ops->mapped)
> > > > + return 0;
> > > > +
> > >
> > > Hello!
> > >
> > > Can vm_ops be NULL here? __compat_vma_mapped() is called from
> > > compat_vma_mmap(), which is reached when a filesystem provides
> > > mmap_prepare. If the mmap_prepare hook does not set desc->vm_ops,
> > > vma->vm_ops will be NULL and this dereferences a NULL pointer.
> >
> > I _think_ for this to ever be invoked, you would need to be dealing with a
> > file-backed VMA so vm_ops->fault would HAVE to be defined.
> >
> > But you're right anyway as a matter of principle we should check it! Will
> > fix.
> >
> > >
> > > For e.g. drivers/char/mem.c, mmap_zero_prepare() would trigger
> > > a NULL pointer dereference here.
> > >
> > > Would need to do
> > > if (!vm_ops || !vm_ops->mapped)
> > > return 0;
> > >
> > > here
> >
> > Yes.
> >
> > >
> > >
> > > > + err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff,
> > > > file,
> > > > + &vm_private_data);
> > > > + if (err)
> > > > + unmap_vma_locked(vma);
> > >
> > > when mapped() returns an error, unmap_vma_locked(vma) is called
> > > but execution continues into the vm_private_data update below. After
> > > unmap_vma_locked() the VMA may be freed (do_munmap can remove the VMA
> > > entirely), so accessing vma->vm_private_data after that is a
> > > use-after-free.
> >
> > Very good point :) will fix thanks!
> >
> > Probably:
> >
> > if (err)
> > unmap_vma_locked(vma);
> > else if (vm_private_data != vma->vm_private_data)
> > vma->vm_private_data = vm_private_data;
> >
> > return err;
> >
> > Would be fine.
> >
> > >
> > > Probably need to do:
> > > if (err) {
> > > unmap_vma_locked(vma);
> > > return err;
> > > }
> > >
> > > > + /* Update private data if changed. */
> > > > + if (vm_private_data != vma->vm_private_data)
> > > > + vma->vm_private_data = vm_private_data;
> > > > +
> > > > + return err;
> > > > +}
> > > > +
> > > > /**
> > > > * compat_vma_mmap() - Apply the file's .mmap_prepare() hook to an
> > > > * existing VMA and execute any requested actions.
> > > > @@ -1191,34 +1240,26 @@ EXPORT_SYMBOL(flush_dcache_folio);
> > > > */
> > > > int compat_vma_mmap(struct file *file, struct vm_area_struct *vma)
> > > > {
> > > > - struct vm_area_desc desc = {
> > > > - .mm = vma->vm_mm,
> > > > - .file = file,
> > > > - .start = vma->vm_start,
> > > > - .end = vma->vm_end,
> > > > -
> > > > - .pgoff = vma->vm_pgoff,
> > > > - .vm_file = vma->vm_file,
> > > > - .vma_flags = vma->flags,
> > > > - .page_prot = vma->vm_page_prot,
> > > > -
> > > > - .action.type = MMAP_NOTHING, /* Default */
> > > > - };
> > > > int err;
> > > >
> > > > - err = vfs_mmap_prepare(file, &desc);
> > > > - if (err)
> > > > - return err;
> > > > -
> > > > - err = mmap_action_prepare(&desc, &desc.action);
> > > > + err = __compat_vma_mmap(file, vma);
> > > > if (err)
> > > > return err;
> > > >
> > > > - set_vma_from_desc(vma, &desc);
> > > > - return mmap_action_complete(vma, &desc.action);
> > > > + return __compat_vma_mapped(file, vma);
> > > > }
> > > > EXPORT_SYMBOL(compat_vma_mmap);
> > > >
> > > > +int __vma_check_mmap_hook(struct vm_area_struct *vma)
> > > > +{
> > > > + /* vm_ops->mapped is not valid if mmap() is specified. */
> > > > + if (WARN_ON_ONCE(vma->vm_ops->mapped))
> > > > + return -EINVAL;
> > >
> > > I think vma->vm_ops can be NULL here. Should be:
> > >
> > > if (vma->vm_ops && WARN_ON_ONCE(vma->vm_ops->mapped))
> > > return -EINVAL;
> >
> > I think again you'd probably only invoke this on file-backed so be ok, but
> > again
> > as a matter of principle we should check it so will fix, thanks!
> >
> > >
> > > > +
> > > > + return 0;
> > > > +}
> > > > +EXPORT_SYMBOL(__vma_check_mmap_hook);
>
> nit: Any reason __vma_check_mmap_hook() is not inlined next to its
> user vfs_mmap()?
Headers fun, fs.h is a 'before mm.h' header, so vm_operations_struct is not
declared yet here, so we can't actually do the check there.
>
> > > > +
> > > > static void set_ps_flags(struct page_snapshot *ps, const struct folio
> > > > *folio,
> > > > const struct page *page)
> > > > {
> > > > @@ -1316,10 +1357,7 @@ static int mmap_action_finish(struct
> > > > vm_area_struct *vma,
> > > > * invoked if we do NOT merge, so we only clean up the VMA we
> > > > created.
> > > > */
> > > > if (err) {
> > > > - const size_t len = vma_pages(vma) << PAGE_SHIFT;
> > > > -
> > > > - do_munmap(current->mm, vma->vm_start, len, NULL);
> > > > -
> > > > + unmap_vma_locked(vma);
> > > > if (action->error_hook) {
> > > > /* We may want to filter the error. */
> > > > err = action->error_hook(err);
> > > > diff --git a/mm/vma.c b/mm/vma.c
> > > > index 054cf1d262fb..ef9f5a5365d1 100644
> > > > --- a/mm/vma.c
> > > > +++ b/mm/vma.c
> > > > @@ -2705,21 +2705,35 @@ static bool can_set_ksm_flags_early(struct
> > > > mmap_state *map)
> > > > return false;
> > > > }
> > > >
> > > > -static int call_action_complete(struct mmap_state *map,
> > > > - struct mmap_action *action,
> > > > - struct vm_area_struct *vma)
> > > > +static int call_mapped_hook(struct vm_area_struct *vma)
> > > > {
> > > > - int ret;
> > > > + const struct vm_operations_struct *vm_ops = vma->vm_ops;
> > > > + void *vm_private_data = vma->vm_private_data;
> > > > + int err;
> > > >
> > > > - ret = mmap_action_complete(vma, action);
> > > > + if (!vm_ops || !vm_ops->mapped)
> > > > + return 0;
> > > > + err = vm_ops->mapped(vma->vm_start, vma->vm_end, vma->vm_pgoff,
> > > > + vma->vm_file, &vm_private_data);
> > > > + if (err) {
> > > > + unmap_vma_locked(vma);
> > > > + return err;
> > > > + }
> > > > + /* Update private data if changed. */
> > > > + if (vm_private_data != vma->vm_private_data)
> > > > + vma->vm_private_data = vm_private_data;
> > > > + return 0;
> > > > +}
> > > >
> > > > - /* If we held the file rmap we need to release it. */
> > > > - if (map->hold_file_rmap_lock) {
> > > > - struct file *file = vma->vm_file;
> > > > +static void maybe_drop_file_rmap_lock(struct mmap_state *map,
> > > > + struct vm_area_struct *vma)
> > > > +{
> > > > + struct file *file;
> > > >
> > > > - i_mmap_unlock_write(file->f_mapping);
> > > > - }
> > > > - return ret;
> > > > + if (!map->hold_file_rmap_lock)
> > > > + return;
> > > > + file = vma->vm_file;
> > > > + i_mmap_unlock_write(file->f_mapping);
> > > > }
> > > >
> > > > static unsigned long __mmap_region(struct file *file, unsigned long
> > > > addr,
> > > > @@ -2773,8 +2787,11 @@ static unsigned long __mmap_region(struct file
> > > > *file, unsigned long addr,
> > > > __mmap_complete(&map, vma);
> > > >
> > > > if (have_mmap_prepare && allocated_new) {
> > > > - error = call_action_complete(&map, &desc.action, vma);
> > > > + error = mmap_action_complete(vma, &desc.action);
> > > > + if (!error)
> > > > + error = call_mapped_hook(vma);
> > > >
> > > > + maybe_drop_file_rmap_lock(&map, vma);
> > > > if (error)
> > > > return error;
> > > > }
> > > > diff --git a/tools/testing/vma/include/dup.h
> > > > b/tools/testing/vma/include/dup.h
> > > > index 908beb263307..47d8db809f31 100644
> > > > --- a/tools/testing/vma/include/dup.h
> > > > +++ b/tools/testing/vma/include/dup.h
> > > > @@ -606,12 +606,34 @@ struct vm_area_struct {
> > > > } __randomize_layout;
> > > >
> > > > struct vm_operations_struct {
> > > > - void (*open)(struct vm_area_struct * area);
> > > > + /**
> > > > + * @open: Called when a VMA is remapped or split. Not called upon
> > > > first
> > > > + * mapping a VMA.
> > > > + * Context: User context. May sleep. Caller holds mmap_lock.
> > > > + */
>
> This comment should have been introduced in the previous patch.
It's the testing code, it's not really important. But if I respin I'll fix... :)
>
> > > > + void (*open)(struct vm_area_struct *vma);
> > > > /**
> > > > * @close: Called when the VMA is being removed from the MM.
> > > > * Context: User context. May sleep. Caller holds mmap_lock.
> > > > */
> > > > - void (*close)(struct vm_area_struct * area);
> > > > + void (*close)(struct vm_area_struct *vma);
> > > > + /**
> > > > + * @mapped: Called when the VMA is first mapped in the MM. Not
> > > > called if
> > > > + * the new VMA is merged with an adjacent VMA.
> > > > + *
> > > > + * The @vm_private_data field is an output field allowing the user
> > > > to
> > > > + * modify vma->vm_private_data as necessary.
> > > > + *
> > > > + * ONLY valid if set from f_op->mmap_prepare. Will result in an
> > > > error if
> > > > + * set from f_op->mmap.
> > > > + *
> > > > + * Returns %0 on success, or an error otherwise. On error, the VMA
> > > > will
> > > > + * be unmapped.
> > > > + *
> > > > + * Context: User context. May sleep. Caller holds mmap_lock.
> > > > + */
> > > > + int (*mapped)(unsigned long start, unsigned long end, pgoff_t pgoff,
> > > > + const struct file *file, void **vm_private_data);
> > > > /* Called any time before splitting to check if it's allowed */
> > > > int (*may_split)(struct vm_area_struct *area, unsigned long addr);
> > > > int (*mremap)(struct vm_area_struct *area);
> > > > @@ -1345,3 +1367,11 @@ static inline void vma_set_file(struct
> > > > vm_area_struct *vma, struct file *file)
> > > > swap(vma->vm_file, file);
> > > > fput(file);
> > > > }
> > > > +
> > > > +static inline void unmap_vma_locked(struct vm_area_struct *vma)
> > > > +{
> > > > + const size_t len = vma_pages(vma) << PAGE_SHIFT;
> > > > +
> > > > + mmap_assert_locked(vma->vm_mm);
> > > > + do_munmap(vma->vm_mm, vma->vm_start, len, NULL);
> > > > +}
> > > > --
> > > > 2.53.0
> > > >
> > > >
> >
> > Cheers, Lorenzo
